In electrophotography, an organophotoreceptor in the form of a plate, disk, sheet, belt, drum or the like having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of the photoconductive layer, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas, thereby forming a pattern of charged and uncharged areas. A liquid or solid toner is then deposited in either the charged or uncharged areas depending on the properties of the toner to create a toned image on the surface of the photoconductive layer. The resulting toned image can be transferred to a suitable receiving surface such as paper. The imaging process can be repeated many times to complete a single image and/or to reproduce additional images.
Both single layer and multilayer photoconductive elements have been used. In single layer embodiments, a charge transport material and charge generating material are combined with a polymeric binder and then deposited on the electrically conductive substrate. In multilayer embodiments, the charge transport material and charge generating material are in the form of separate layers, each of which can optionally be combined with a polymeric binder, deposited on the electrically conductive substrate. Two arrangements are possible. In one arrangement (the “dual layer” arrangement), the charge generating layer is deposited on the electrically conductive substrate and the charge transport layer is deposited on top of the charge generating layer. In an alternate arrangement (the “inverted dual layer” arrangement), the order of the charge transport layer and charge generating layer is reversed.
In both the single and multilayer photoconductive elements, the purpose of the charge generating material is to generate charge carriers (i.e., holes and/or electrons) upon exposure to light. The purpose of the charge transport composition is to accept holes, i.e., positive charge carriers, and to transport them through the charge transport layer in order to facilitate discharge of a surface charge on the photoconductive element.
As more advanced, higher speed electrophotographic systems such as copiers, duplicators, fax machines, and printers were developed, degradation of image quality was encountered during cycling. Moreover, complex, highly sophisticated electrophotographic systems operating at high speeds have placed stringent requirements including narrow operating limits on organophotoreceptors. For example, the numerous layers found in many modern organophotoreceptors must adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles.